If I were to upgrade the NVR's firmware to the latest 3.4.96, would I run into a language mismatch problem with my grey market cameras? It appears it's only a few months newer than what I already have, 171117 vs. 170818.

I'm pretty sure that the NVR firmware 3.4.92 already has the camera language check and block built in - it was introduced much earlier.

If the cameras are running 'hacked to English' firmware they could still manifest as EN such that the NVR will be OK with them.
A couple of things to test :
Enable telnet/SSH on the cameras and check the language value in the prtHardInfo command.
1=EN, 2=CN
Point the browser at the cameras with (adjust IP address to suit)http://192.168.1.101/SDK/Language
Depending on what the cameras are running, the 2 results may differ.